What sort of fork are you hoping to use? Carbon with a carbon steerer?
If it's an old bike, you'll most likely need a 1" wide steerer; most of the new carbon stuff is 1"1/8
What sort of head-set will you use?
This is can obviously vary, depending on the head-set, but if you have a "modernish" threaded Shimano head-set, the way I've just measured mine, the steerer needs to be AT LEAST 35mm longer than the head tube. This not ideal, because you won't have any spacers under the lock nut.
If you get a long steerer, you need to make sure the thread is long enough......it goes on and on, so it all depends on what you're gunna use.
I'm no mechanic, so I'm sure Bourdreaux will correct me -- I give him half an hour😄